Neil Lupin/RedfernsPills that investigators seized from Prince‘s Paisley Park compound that were labeled as hydrocodone contained the powerful opioid fentanyl, a source familiar with the investigation told the Minneapolis Star Tribune.

It was fentanyl that killed Prince, according to autopsy results. He did not have a prescription for the drug, the source said.

Investigators do not know at this point how Prince ingested the fentanyl. However, they believe he may have taken the pills marked as hydrocodone not knowing they contained fentanyl, according to the Star Tribune.

Prince weighed only 112 pounds when he died, but he had so much fentanyl in his system that no one would have survived, the source revealed.

Sources with knowledge of the investigation have told the newspaper that autopsy results also revealed the presence of the anesthetic lidocaine, alprazolam – more commonly known as the sedative Xanax — and the painkiller Percocet.

Prince was found dead at his Paisley Park compound April 21. He was 57.

Copyright © 2016, ABC Radio. All rights reserved.